Myocardial inflammation occurs in in response to ischemic injury, infections, and activation of autoreactive or alloreactive lymphocytes. Myocardial function is impaired by inflammatory mediators, and inflammation contributes to chronic myocardial injury, fibrosis, and heart failure. Innate and adaptive immune responses are tightly regulated in the heart, which serves to prevent maladaptive inflammation. Nonetheless, these mechanisms are poorly understood, and when they fail, inflammatory responses are central to the pathogenesis of myocardial disease. A better understanding of the normal mechanisms that suppress myocardial inflammation is key to the design of therapeutic strategies to limit inflammatory damage in the setting of several cardiac diseases. Extensive preliminary work has demonstrated that the molecules PD-1 and its ligand PD-L1 are important regulators of adaptive immune responses to microbial, alloreactive and self- antigens. In particular, the PD 1/PD L1 pathway appears to be a central negative regulatory checkpoint in controlling immune responses in the heart. However, because of the complexities of the cellular expression of PD-1 and PD-L1, it is still unclear how the pathway is actually engaged to suppress myocardial inflammation. The broad objectives of this project are to discover how cardiac inflammation is regulated by endothelial, myocyte, and dendritic cell PD-L1, and by myeloid PD-1. The work will take advantage of newly developed mouse lines with lineage specific deficiencies of PD-L1 and PD-1, which will be used in the context of models of innate and adaptive immune mediated myocardial inflammation. These approaches will be organized into the following three Specific Aims: Aim 1. Determine which cell types mediate PD-L1-dependent protection against T cells in the heart, the mechanisms of protection, and the feasibility of therapeutically engaging these mechanisms. Aim 2. Test the hypothesis that the PD-1-PD-L1 pathway directly regulates myeloid cell inflammatory responses in the heart. Aim 3. Develop PD-1:PD-L1 dependent methods to therapeutically induced T cell tolerance to myocardial antigens The data generated by the completion of these Aims will define for the first time the regulation of cardiac inflammation by endothelial, myocyte, and DC PD-L1, and by myeloid PD-1. The findings will be important for development of therapeutic modalities to treat myocardial inflammation and for an understanding of the risks of therapeutic approaches to block PD-1 and PD-L1, which are actively being developed in cancer and chronic viral infection.